References here to “flat steel products” are to steel strips, steel sheets, or blanks and the like obtained from them.
In order to offer the combination of low weight, maximum strength, and protective effect that is required in modern-day bodywork construction, components hot-press-formed from high-strength steels are nowadays used in those areas of the bodywork that may be exposed to particularly high loads in the event of a crash.
In hot press hardening, also called hot forming, steel blanks provided from cold-rolled or hot-rolled steel strip are heated to a forming temperature, which is generally located above the austenitization temperature of the respective steel, and are placed in the heated state into the die of a forming press. In the course of the forming that is subsequently carried out, the sheet blank or the component formed from it undergoes rapid cooling as a result of contact with the cool die. The cooling rates here are set so as to produce hardened microstructure in the component.
One typical example of a steel suitable for hot press hardening is known by the designation “22MnB5” and can be found in the 2004 German steel codex (Stahlschlüssel) under material number 1.5528.
In practice, the advantages of the known manganese-boron steels which are particularly suitable for hot press hardening are balanced by the disadvantage that, generally speaking, manganese-containing steels are unstable toward wet corrosion and are difficult to passivate. This tendency, which is strong by comparison with less highly alloyed steels, on exposure to elevated chloride ion concentrations, toward corrosion which, while locally limited, is nevertheless intensive makes it difficult for steels belonging to the high-alloy steel sheet materials group to be used, particularly in bodywork construction. Moreover, manganese-containing steels have a tendency toward surface corrosion, thereby likewise restricting the spectrum of their usefulness.
It is known from investigations, moreover, that in the case of temperable Mn—B steels for complex, crash-critical structural components in vehicle bodies, under adverse conditions, as for example on increased hydrogen introduction and in the presence of elevated tensile stresses, during the fabrication or the further processing of these steels, there is potentially a risk of hydrogen embrittlement and/or of the incidence of delayed, hydrogen-promoted cracking. The introduction of hydrogen is favored by the relatively high accommodation capacity of the steel substrate in the austenitic microstructure state during the annealing treatment.
Various proposals exist in the prior art aimed at reducing the hydrogen absorption of manganese-containing steels during the tempered state and/or else at providing such steels with a metallic coating that protects the steel from corrosive attack. Distinctions are made between active and passive anticorrosion systems.
Active anticorrosion systems are customarily produced by continuous application of a zinc-containing anticorrosion coating. Passive anticorrosion systems, in contrast, are produced typically by application of an aluminum-based coating which affords a good barrier effect to corrosive attacks.
Known metallic, zinc-containing anticorrosion coatings have negative and positive aspects.
In a method known from EP 2 290 133 A1 for producing a steel component provided with metallic corrosion protection, a zinc-nickel alloy coating is deposited electrolytically on a steel strip. The strip speeds possible are low as a result of the electrolytic coating operation, and this raises the production costs. As a result of intercalated zinc phases, however, active corrosion protection after hot forming (hot press hardening) is ensured. In comparison to zinc-containing anticorrosion coatings produced by hot dip coating, there is a greater welding range for spot resistance welding according to standard parameters of SEP 1245.
Steel blanks with zinc-based anticorrosion coatings that have a high zinc fraction in the range of at least 85 wt % and at most 98 wt % and that are applied by means of hot dip coating lines to a steel strip that is to be coated can be processed only in a costly indirect hot forming operation. The SEP 1245 welding ranges for spot resistance welding that are achievable in the case of steel components with zinc-based anticorrosion coatings of this kind, after hot forming, are at a very low level. As a result of the relatively low melting temperature of this coating material, low bath temperatures are operated, and this lowers, or makes relatively favorable, the production costs for the zinc-based coating in comparison to those for an AlSi coating. With zinc-based coatings of this kind, however, their low melting temperature results in a high risk of zinc-infiltrated cracking (liquid phase embrittlement). The oxide phases (generally aluminum oxide) that are positive for the evaporation behavior, moreover, are typically deliberately removed at the surface after press hardening. As a result, a substantially pure zinc-iron coating is present on the press-hardened component.
In the likewise-known galvannealing process, the zinc coating on a steel strip is converted by means of an additional annealing step into a zinc-iron alloy layer, in order to raise the concentration of iron in the coating to more than 40 wt %. But zinc coatings of this kind, enriched with iron for direct hot forming, have active corrosion protection only during a short processing window in hot forming. Components which have been heated for too long no longer provide active corrosion protection. Process times that are too short may result, as in the case of other zinc-based anticorrosion coatings, to zinc-infiltrated cracking, in turn. The oxide phases (generally aluminum oxide) that are positive for the evaporation behavior, moreover, are typically removed at the surface after press hardening in the case of the galvannealing process as well. As a result, a substantially pure zinc-iron coating is present on the press-hardened component.
With existing aluminum-based anticorrosion coatings as well there are a number of adverse aspects. For instance, the energy consumption of a hot dip coating line for producing AlSi coatings is relatively high, owing to the high melting temperature of the coating material. Furthermore, on manganese-boron steels, these coatings can be cold-formed only to a certain extent. On account of a hard intermetallic Fe—Al—Si phase, the cold-forming operation is accompanied by instances of flaking of the coating. As a result, degrees of forming are restricted. In general, therefore, the AlSi coatings require direct hot forming. In combination with a cathodic electrodeposition coating, which allows the coating film to adhere well to the surface of the AlSi coating, a good barrier effect with respect to corrosive attacks can be obtained. With this coating variant, moreover, it is necessary to consider the introduction of hydrogen into the steel material, which may necessitate the use of dew point regulation in the continuous oven for the press hardening process if process conditions are adverse. The energy consumption associated with dew point regulation gives rise to additional costs in component manufacture.
Therefore, a need exists for methods that can be carried out simply in practice and that allow for the production at comparatively low cost and complexity of steel components having well-adhering metallic coatings that protect against corrosion.